There is a speaker in which an opening is formed in a bobbin of a voice coil. Such a speaker according to a related art will be described with reference to a side sectional view of FIG. 4.
Referring to FIG. 4, magnetic circuit 21 is configured by adhesively coupling plate 22, magnet 23, and yoke 24. Voice coil 26 is supported by damper 27 so as to be moved up and down. Vibrating plate 28 is formed such that an inner circumference is coupled to voice coil 26 and an outer circumference is coupled to frame 25. Dust cap 29 for dustproofing is disposed in the center of vibrating plate 28. A bobbin of voice coil 26 has a plurality of openings 30 formed therein. When openings 30 reduce air spring in an inner space formed by voice coil 26, magnetic circuit 21 and dust cap 29, this allows the vibrating system to easily move.
Further, the influence of an acoustic circuit generated in the inner space is reduced to improve the sound quality such as articulation. Further, Japanese Patent Unexamined Publication No. 59-72298 is an example of prior art relating to this invention.
According to the above publication, in order to improve the operation of a speaker in which a magnetic fluid is filled in a magnetic gap, openings are provided in the bobbin of the voice coil. However, the openings are different from a typical bobbin opening.
Any of the typical bobbin opening shown in FIG. 4 and the bobbin opening disclosed in the above publication reduces not only the influence of the acoustic circuit inside the voice coil and the air spring during vibration, but also the air resistance. A full range speaker uses the air resistance as part of a damping force. Therefore, if the air damping is insufficient, abnormal sounds may frequently be generated during large vibrations. As a result, it is difficult to provide openings in the bobbin of the voice coil in order to reduce the influence of the acoustic circuit inside the voice coil in the speaker that uses air damping.